Preliminary experiments in our laboratories have shown that electrical current (taken as the movement of positive charge) leaves the hind limb bud of Xenopus larvae. This current begins before any obvious morphological signs of the limb bud appear and persists throughout development. This current is detected with the vibrating probe, which measures voltages produced by the current (via Ohm's law) in the medium surrounding the larvae. This current has two sources: the general epithelium surrounding the limb bud in frog larvae (later in development), the gills. It is also found that the vitamin A analog, retinoic acid, can cause a reversal of this current and a rapid degeneration of a new limb bud. Thus, it is proposed that the well-known regulatory role of the epithelium covering the limb bud is due, at least in part, to it becoming ionically less tight, and so, electrically leaky, such that current pumped in elsewhere leaks out at the limb. We propose that the natural sub-epidermal voltage gradients produced in the local of the electrical leak provide a vector for the early accumulation of mesenchyme cells and, perhaps, later innervation of the developing bud. Work to be done under this proposal includes a careful and complete characterization of the electrical currents associated with limb development. The temporal, spacial, and ionic components will be determined. An effort will be made to determine causal relationships by modulating the current and looking for effects on limb development. We will extend and expand our efforts at correlating this early physiology of the pre-bud and limb forming flank with our observations of progressive changes in the ultrastructural anatomy of the limb and prelimb integument. The study will be expanded to include avian embryos in order to establish the generality of the phenomenon and to make use of many useful "limb" mutants available in the chick system. The possibility that thalidomide exerts its teratological effect via the epithelium will be explored. The health-related aspect of this proposal is that it seeks to understand one aspect of how limbs develop normally and some causes of aberrant development.